Not only is technology making physical school textbooks "a thing of the past," the Gateses wrote, but it's making education a whole lot cheaper, too.

Modern software not only gives students access to text online, but can identify the topics students are struggling to grasp and generate quiz questions on them. Teachers now get reports of their students' online performances and can know which subjects to focus on the following day.

Bill Gates cited the online curricula Zearn, i-Ready, and LearnZillion, as well as a free digital course he funds called Big History, as examples of software that is eliminating the need for physical textbooks.

"I hope this growing momentum will inspire more of the big textbook publishers, which have been slow to offer these kinds of tools," Bill Gates wrote.

In college classrooms, digital courseware can save students as much as $121 per course, or about $1,000 a year, Melinda Gates said in the letter. She also cited a study that said students who use digital learning tools for introductory college classes performed better than those who learned in classroom settings.

That's great news for students who don't fit the traditional stereotype — that is, starting college right after high school, living in a dorm, and graduating in four years. Rather, Melinda Gates said almost half of college students in the US today are 25 or older, while more than half have a job and more than a quarter have children.

"Not having to show up to a physical classroom at a specific time makes a big difference to students who are balancing school with working and raising a family," Melinda Gates wrote.

"We now have the tools to redesign higher education so that it meets the needs of today's students."